The present invention concerns a device for embolism detection as set forth in the classifying portion of claim 1, and a method of embolism detection.
Such devices and methods of the general kind set forth are used in medical ultrasound diagnostics in order on the basis of the reflected ultrasound Doppler signals and in particular changes therein to be able to draw conclusions about foreign bodies which are disposed in a blood vessel. It is known that in particular embolisms or comparable foreign bodies are distinguished by a reflection characteristic in respect of the introduced ultrasonic signal, that is greatly different in comparison with a surrounding fluid mediumxe2x80x94for example bloodxe2x80x94, so that these particularities are used for embolism detection.
In that respect, the state of the art includes devices which, by suitable demodulation of the received reflected Doppler signal and subsequent acoustic output afford an operatorxe2x80x94for example a doctor dealing with the casexe2x80x94an acoustic way of detecting an embolism. The latter is expressed in the acoustic output signal by virtue of a characteristic noise.
In addition the state of the art discloses apparatuses which also prepare an ultrasonic signal reflected from a flow of blood, for optical display and evaluation. Those apparatuses are distinguished in that a received ultrasonic signal which is reflected at the flow of blood in a vessel is demodulated and subsequently prepared by means of digital image processing in such a way that a spectral representation of the ultrasonic signal (of the motion signal) in relation to time can be obtain for example on a surveillance monitor. Especially provided processors implement in that respect the necessary steps for image generation, in particular Fourier transformation of the received data. In that way then a detected foreign body would be visually represented on the display screen, for example by virtue of coloured emphasis of the characteristic signal amplitude of the embolism signal in the surrounding flow of blood in the spectral representation.
In practical operation however it has been found useful to effect the detection of foreign bodies in the blood stream in a more precise manner and in particular also to make it possible to distinguish embolisms from so-called artefacts, more specifically signal disturbances of an ultrasonic probe which is used for the methods and devices of the general kind set forthxe2x80x94as occur for example due to movements of the probe; more specifically, just like an embolism, an artefact results in a characteristic signal change in an optical or acoustic output signal of the device and, in the case of an artefact, would impair diagnostic accuracy and result in the operator being unnecessarily distracted.
The specific task of distinguishing an embolism or the like foreign body in the bloodstream from an artefact was approached in various ways in the state of the art. Thus for example the teaching of U.S. Pat. No. 5,103,827 makes use of the properties of an artefact (in comparison with an embolism) that, in the case of an artefact, the image representation or display involves a bi-directional spectral signal (both in a positive and also a negative direction) which can be distinguished from a uni-directional embolism signal by virtue of suitable circuitry measures and measures relating to signal processing procedures. However such a solution, due to the necessary variable thresholds for distinction purposes and the complication and expenditure which this entails, is only limitedly suitable for affording a simple and convenient way of distinguishing embolisms. In addition it is precisely in relation to relatively large embolisms that the problem arises thatxe2x80x94for example due to over-driving of interposed amplifier unitsxe2x80x94such relatively large embolisms also result in a bi-directional spectral representation so that in that respect the path adopted cannot in any case lead to a satisfactory result.
In addition U.S. Pat. No. 5,348,015 describes a further approach for distinguishing an artefact from an embolism. In particular the inventors here propose using a multi-channel device which is operated at various frequencies for embolism detection and distinction. As more specifically the ultrasonic reflection properties in particular of an embolism are frequency-dependent, that affords a secure way of distinguishing same for example from an artefact (which is uninfluenced thereby). More specifically, in the case of an embolismxe2x80x94in contrast to an artefactxe2x80x94a signal (amplitude) strength which is different for various ultrasonic frequencies is to be established in the spectral representation, and that strength can then be evaluated. It will be noted however that the device described in U.S. Pat. No. 5,348,015 is extremely complicated and expensive and, besides a plurality of transmitting and receiving channels (that is to say signal generation, reception and demodulation are respectively required separately), the device also requires special probes which are suitable for a multi-frequency mode of operation, and in addition it gives rise to considerable difficulties in regard to control and software engineering. In particular from the point of view of inexpensive and uncomplicated implementation of reliable embolism detection and distinction therefore, this approach also appears to suffer from disadvantages.
The object of the present invention is therefore that of improving a device for embolism detection of the general kind set forth, in such a way that detection of an embolism and in particular distinction thereof from an artefact can be effected in a more reliable and simpler fashion. The invention also aims to provide a suitable embolism detection method.
That object is attained by the device having features of claim 1 and the method as set forth in claim 9.
The device according to the invention advantageously makes it possible to reliably distinguish an embolism from an artefact on the basis of two signals which are evaluated in respect of their respective signal changesxe2x80x94which could indicate an embolismxe2x80x94and the time interval between those signal changes.
In that respect the invention makes use of the principle that an artefact, produced for example by movement of the ultrasonic probe in the position of attachment to the head and as an interference signal in the two signals to be evaluated occurs substantially simultaneously or however only with a minimum time interval. In comparison an embolism is made distinguishable by virtue of the fact that, when reaching the first position in the (blood) vessel, it produces the characteristic signal change for example arise in amplitudexe2x80x94while this still cannot be the case at the second position which is different from the first position, at that moment in time. On the contrary, at the second position, if this is also in the same vessel, the characteristic signal change occurs in respect of time prior to or after the first signal change, in dependence on how the first and second positions are arranged relative to each other in the direction of flow of the fluid (the embolism moves with the speed of flow of the fluid in the vessel) so that the time interval between the respective signal changes corresponds to the duration of transportation between the two positions. If the second position is arranged outside the vessel, then in the normal case no characteristic signal change in the second signal is produced by the embolism in the vessel.
In that respect, in connection with the invention, the ultrasonic transmitting device may have one or a plurality of ultrasonic probes.
Advantageous developments of the invention are set forth in the appendant claims.
Thus it is particularly preferred for the second position to be arranged outside the vessel to be monitored; in a further preferred feature that positionxe2x80x94when implementing monitoring on the head of a patientxe2x80x94is at a depth of between about 30 and about 35 mm in relation to the surface of the head and directed onto a skull bone.
In that way there is then practically no change in the second signalxe2x80x94that is to say in the reference channelxe2x80x94when an embolism passes the first position and thereupon the first characteristic signal is generated.
It is also to be assumed that the minimum time interval in use in a practical medical context is practically zero and even theoretically does not exceed between 2 and 3 msec.
Advantageously there is provided a so-called gating system for embolism detection according to the invention, more specifically evaluation, displaced in respect of time, of the same reflected transmitting signal, whereby observation of two different depths of penetration in the body is made possible. In accordance with the invention a first depth of penetration is set to the first position while preferably the depth of penetration which determines the second position is outside the vessel. It is also in accordance with the invention to provide a plurality of gatesxe2x80x94which are for example graduated or stepped in terms of the depth of penetrationxe2x80x94and of which then at least one is to be used as a reference gate to be employed to generate the second signal.
While moreover operation of the detector unit is usually implemented on the basis of first and/or second signals which are transformed into the time domain (that is to say FFT), embolism detection and distinction is in principle also possible in the time domain in accordance with the invention; it is advantageously possible in that way to save on an FFT-processor at least for the reference channel.
Further advantages, features and details of the invention will be apparent from the description hereinafter of specific embodiments and with reference to the drawings in which: